In the manufacture of foundry molds and cores from sand using a curable binder, recycling the sand is an important economic consideration. Foundry sand can normally be used, then reused, repeatedly, when most resin binders are used. Normally there is a small loss of sand on reuse, and this loss is made up by adding pristine sand to the used sand. These conditions are experienced with most resin binders.
In recent years, however, ester cured alkaline phenolic resins have gone into widespread use. These resins can be ester cured at ambient temperature. Such resin binder systems are disclosed, for example, in patents: U.S. Pat. No. 4,426,467, in which lactones are used as the curing agents; U.S. Pat. No. 4,474,904, in which esters are so used; and U.S. Pat. No. 4,468,359, in which the esters are in the gaseous or vapor phase.
While these binding systems offer many advantages, there are offsetting disadvantages that are sometimes observed. For example when the sand is reused, the tensile strength of the molds or cores drops off. This may have a serious effect on the economics of the foundry.
The extent to which previously used sand is able to be reused is often determined by the tensile properties that can be achieved. One factor which determines the tensile strength is the ability of the binder to bond to the surfaces of the reclaimed sand. Higher rebonding strengths allow higher usage levels of reclaimed sand.
Sand reclamation is the physical, chemical or thermal treatment of a refractory aggregate to allow its reuse. Ideally there is no significant loss of its original useful properties as required for the application involved.
In typical foundry operations, the sand is collected after taking out a casting from a poured mold. This sand comprises loose sand grains, sand agglomerates and lumps of bonded sand. All these are broken down by mechanical devices into free-flowing granules. These granules are screened to produce reclaimed sand, ready for reuse. This is the simplest form of sand reclamation. The reclaimed sand from this process generally has layers of burned and partially burned binder films still adhering to it. The amount of such organic layer present can be determined by a loss-on-ignition (L.O.I.) determination.
The granular sand can be further processed to remove the binder residue layer either by mechanical means (sand scrubbers) or thermal means (a rotary kiln). The reclaimed sand that has been subjected to these additional processes is characterized by low LOI values as compared to the simple granular sand described above, as it comes from a used mold or core.
Pristine foundry sand would normally have negligible LOI and a particular, specified screen distribution (AFS Handbook). Ideal reclamation would return used sand to its original LOI and screen distribution values.
There are thus three treatments available for reclaiming sand. These include mechanical, wet, and thermal treatment processes. The mechanical treatment process typically involve subjecting the sand granules to grinding, scrubbing or other means of mechanical attrition to provide particles of a desired size, remove binder residues, provide new sand surfaces and/or remove fines. The equipment and process used may depend on the particle size and uniformity desired. The wet treatment processes involve washing the sand granules with water, draining the wash water and drying the washed sand. In thermal treatment processes, the sand is heated to a temperature of about 120.degree. C. or above, wherein the binder residue is decomposed or burned.
The mechanical treatment processes and thermal treatment processes have not proven very effective in enhancing the bonding properties of reclaimed sand obtained from foundry cores and molds wherein the binder is an ester cured alkaline phenolic resin. Such sand is characterized by binder film residues or the sand surfaces, whose presence is indicted by many resulting properties, including a residual alkalinity. These residues may be the cause of the poor bonding characteristics of the reclaimed sand when reused with an ester cured alkaline phenolic resin binder. Wet treatment processes have been found to increase the bonding strength of such reclaimed sand, but are expensive, consume energy, and entail a disposal problem. It is believed the wet treatment processes "cleanse" the surface of the sand of at least some harmful residues, whereas the mechanical and thermal treatment processes are not as effective.
Most foundries utilize a simple mechanical treatment in reclaiming sand, and avoid the environmental problems of wet treatment and the expenditures of energy required for thermal treatment. These foundries compensate for the loss in bonding properties by mixing the reclaimed sand with pristine sand. The usage level of mechanically treated, thermally treated or untreated reclaimed sand may only be about 50 weight percent of the total sand used. There is an economic incentive, as well as environmental and other incentives, to enhance the usage level of such reclaimed sand.
In comparison, sand recovered from foundry molds and cores: which employ an acid cured binder, such as acid cured phenolic resins, acid cured furan resin, and phenolic-urethane resins, generally show no significant loss in bonding strength after thermal or mechanical treatment, as compared to pristine sand. When reclaimed sand shows no loss in bonding ability, generally its reuse can be as high as 80% to 90% by weight of the total sand used. Theoretically, the usage level of such reclaimed sand can be as high as 100%; however, there are usually losses in foundry operations due to spillage and other handling losses. Therefore, it is necessary to replenish the stock of sand with pristine sand.
It is desirable to enhance the bonding affinity and ability of used sand, reclaimed from foundry use, that has been bonded with an ester cured alkaline phenolic resin, particularly a highly alkaline phenolic resin, to the extent that usage levels of such reclaimed sand may be as high as 60% to 80% or 90% by weight in subsequent foundry operations.
Solutions of alkaline phenolic resins that are curable with curing agents having ester functionality have been disclosed in U.S. Pat. Nos. 4,426,467, 4,468,359 and 4,474,904, as mentioned above. In U.S. Pat. No. 4,426,467, the curing agents are lactones, which are self condensed cyclic esters, and the resin binder solutions have preferred solids contents in the range from 80% to 95% by weight. Suitable solids concentrations are said to be in the range from 60% to 95% by weight. The solutions of curable resin binders disclosed in U.S. Pat. Nos. 4,468,359 and 4,474,904 have solids contents falling within the range of 50% to 75% by weight. These three U.S. Patents are expressly incorporated herein by reference.
Resin solutions with lower solids contents in the range from 40% to 70% by weight have been disclosed in Japanese Laid Open Patent Application No. 62-282743, assigned to Dainippon Ink. However, the resin solids in these solutions are augmented with added resin solids prior to use. These added resin solids are dissolved in the organic ester curing agent in an amount of from 20% to 60% by weight, based on the weight of the ester.
It is recognized in the art that the tensile strengths of the finished products suffer when the solids content of the binder solution falls below about 50% by weight, when used with pristine sand.